Sodium chlorate is generally prepared by the electrolysis of sodium chloride wherein the sodium chloride is electrolyzed to produce chlorine, sodium hydroxide and hydrogen. The chlorine and sodium hydroxide are immediately reacted to form sodium hypochlorite, which is then converted to chlorate and chloride under controlled conditions of pH and temperature.
Thus, within the electrolytic system, sodium chloride is, in effect, combined with water to form sodium chlorate and hydrogen gas. The electrolysis takes place typically at 60.degree. C. to 90.degree. C. in electrolytic cells comprising anodes, which may be a precious metal or metal oxide coated titanium, and cathodes, which may be steel.
It is common practice in the electrolysis of brines in the production of halates, such as sodium chlorate, to add a chromate (chromium VI), usually in the form of dichromate, typically sodium dichromate, to the electrolyte as a means of improving the overall efficiency. The chromate prevents loss of current efficiency at the cathode, reduces side reactions and improves the conversion of the hypohalite to the halate, e.g. hypochlorite to chlorate. The chromium VI is not consumed by the process and leaves with the metal halate/brine solution.
Chlorate manufacturing plants situated close to an associated, or dedicated, chlorate consuming process will frequently produce a liquor product containing the chlorate. Merchant chlorate plants serving many customers distributed over a wide area typically produce chlorate as a crystalline product. In this way, shipping costs are minimized and loss of residual chromium ion in the liquor is eliminated.
The sodium chloride salt used to prepare the brine for electrolysis to sodium chlorate generally contains impurities which, depending on the nature of the impurity and certain of the production techniques employed, can give rise to plant operational problems which are familiar to those skilled in the art. The means of controlling these impurities are varied and include purging them out of the system into alternative processes or to the drain, precipitation by conversion to insoluble salts, or crystallization or ion exchange treatment. The control of anionic impurities presents more complex problems than that of cationic impurities.
Sulphate ion is a common ingredient in commercial salt. When such salt is used directly, or in the form of a brine solution, and specific steps are not taken to remove the sulphate, the sulphate enters the electrolytic system. Sulphate ion maintains its identity under the conditions in the electrolytic system and thus accumulates and progressively increases in concentration in the system unless removed in some manner. In chlorate plants producing a liquor product, the sulphate ion will leave with the product liquor. In plants producing only crystalline chlorate, the sulphate remains in the mother liquor after crystallization of the chlorate, and is recycled to the cells. Over time, the concentration of sulphate ion will increase and adversely affect electrolysis and cause operational problems due to localized precipitation in the electrolytic cells.
U.S. Pat. No. 4,702,805, Burkell and Warren, issued Oct. 27, 1987, describes an improved method for the control of sulphate in an electrolyte stream in a crystalline chlorate plant, whereby the sulphate is crystallized out. In the production of crystalline sodium chlorate according to U.S. Pat. No. 4,702,805, sodium chlorate is crystallized from a sodium chlorate rich liquor, and the crystals are removed to provide a mother liquor comprising principally sodium chlorate and sodium chloride, together with other components including sulphate and dichromate ions. A portion of the mother liquor is cooled to a temperature to effect crystallization of a portion of the sulphate as sodium sulphate in admixture with sodium chlorate. The crystallized admixture is removed and the resulting spent mother liquor is recycled to the electrolytic process.
It has been found subsequently, that the crystallized admixture of sulphate and chlorate obtained from typical commercial liquors according to the process of U.S. Pat. No. 4,702,805 may be discoloured yellow owing to the unexpected occlusion of a chromium component in the crystals. The discolouration cannot be removed by washing the separated admixture with liquors in which the crystallized sulphate and chlorate are insoluble. This represents a limitation to the process as taught in U.S. Pat. No. 4,702,805. It will be appreciated that the presence of chromium in such a sulphate product could be detrimental in subsequent utilization of this product.
Dichromate used in chlorate manufacture is an expensive chemical and, although processes have been developed for removal of dichromate from chlorate-containing liquors, such processes for the removal of the dichromate from the liquor prior to removal of the sulphate would not be economical or efficient for the overall operation of the electrolytic process.
U.S. Pat. No. 4,636,376, of Maloney and Carbaugh, discloses removing sulphate from aqueous chromate-containing sodium chlorate liquor without simultaneous removal of significant quantities of chromate The chromate and sulphate-containing chlorate liquor having a pH in the range of about 2.0 to about 6.0 is treated with a calcium-containing material at a temperature of between about 40.degree. C. and 95.degree. C., for between 2 and 24 hours to form a sulphate-containing precipitate. The precipitate is predominantly glauberite, Na.sub.2 Ca(SO.sub.4).sub.2.